Semiconductor image sensors are used to sense radiation such as light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charged-coupled device (CCD) sensors are widely used in various applications such as digital camera or mobile phone camera applications. These devices utilize an array of pixels in a substrate, including photodiodes and transistors, to absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals.
A backside-illuminated (BSI) image-sensor device is one type of image-sensor device. The BSI image-sensor device is used for sensing a volume of light projected towards a backside surface of a substrate (which supports the image sensor circuitry of the BSI image-sensor device). The pixel grid is located at a front side of the substrate, and the substrate is thin enough so that light projected towards the backside of the substrate can reach the pixel grid. The BSI image-sensor device provides a high fill factor and reduced destructive interference, as compared to frontside-illuminated (FSI) image-sensor devices. Although existing BSI image-sensor devices and methods of fabricating these BSI image-sensor devices have been generally adequate for their intended purposes, as device scaling down continues, they have not been entirely satisfactory in all respects.